Re-hydration from previous state

[pocesar]

Right now, I'm accessing the "private" members directly, and assigning them manually. would be good to have an static method, like fromJSON(theOneReturnedFromtoJSON) to initialize the classes using a previous state.

[fieldju]

This sounds reasonable. I really dislike the method name toJson that all the metrics impl, because some of them do NOT return JSON, should really be getValue() or getPayload().

Since this lib is inspired from Dropwizard see: https://github.com/dropwizard/metrics/blob/4.1-development/metrics-core/src/main/java/com/codahale/metrics/Gauge.java#L25

So maybe the static initializer could be fromValue.

[pocesar]

recreating the internal representation is harder than I expected, there are too many moving parts (new class instances, and their internals as well)